The present invention relates to a flexible hose suitable for use in particular for transporting fluids under pressure, especially hydrocarbons produced by operating a subsea well.
The Applicant company manufactures and sells great lengths of various types of such hoses, having high mechanical qualities, in particular tensile strength, and resistance to crushing, to the internal pressure of the fluid being transported, and to the effects of twisting.
Thus, the Applicant company manufactures and sells hoses designed to withstand high pressures, greater than 100 bars and possibly as great as 500 bars to 1000 bars, and comprising an internal leakproof sheath, so-called "tension" armoring constituted by at least two crossed layers of wires of simple shape in section, generally rectangular or circular and, between the internal leakproof sheath and the tension armoring, a pressure-resistant armoring referred to as a pressure arch, and comprising one or more layers of metal wires that are spiral-wound at an angle that is close to 90.degree. relative to the axis of the flexible hose, the tension armoring being covered by an outer protective sheath.
In practice, the angle at which the pressure arch is spiral-wound is greater than 80.degree. relative to the axis of the flexible hose so as to give the arch maximum strength against the circumferential component (hoop stress) of the forces generated by the pressure inside the hose.
Various configurations for such pressure arches have already been described, in particular in document WO-91/00467.
That prior document discloses, in particular, a pressure arch configuration constituted by a helical winding of at least one pair of interlockable metal section wires, one of the section wires having a so-called "T-shaped" section, the other section wire having a so-called "U-shaped" section, the wires being organized and dimensioned so that on being wound they provide lateral interlocking of the turns, i.e. limitation of the extent to which the turns can move apart laterally in the direction of the axis of the flexible tubular hose.
A T-section wire can be defined as having a base portion provided at its lateral extremities with ribs projecting from the base portion, and the section also has a central rib projecting from the base portion with the end thereof being located at a greater distance from the base portion than are the ends of the lateral ribs.
The U-section wire comprises a base portion provided at its lateral extremities with ribs that project from the base portion.
In the configurations known from the above-mentioned prior document in which the pressure layer is made by spiral-winding and interlocking section wires where one has a T-shaped section and the other has a U-shaped section, the U-section wire is large in thickness and in section.
According to that prior document, the U-section wire can be disposed either with its ribs facing outward or else with its ribs facing inward.
As a result of experimental work, the Applicant company has found on the one hand that in dynamic applications, i.e. when the flexible hose is subjected to alternating bending deformation, such a flexible hose presents very disappointing performance if it has a pressure layer constituted by an inner layer of U-section wire and an outer layer of T-section wire, so that the U-section wire has ribs that are outwardly directed. It has been observed that, in practice, once the hose is in the assembled state, there always exists contact between the T- and U-section wires constituting the pressure layer and that this continues throughout the time the hose is in use, with the mutual thrust forces between the projecting portions of the section wires being large.
It has been observed that, in use, the friction generated by such contact, associated with the large thrust forces, gives rise to a reduction in lifetime due to cracking phenomena that appear in the section wires and due to the reduction of the section thereof.
On deciding to make the inner layer out of T-section wire with the ribs of the U-section wire being inwardly directed, it has been found that, in practice, it is not possible to prevent radial contact taking place between the T-section wire and the U-section wire, and that the U-section wire contributes significantly to the resistance to the circumferential component of the pressure forces. Under such conditions, the bearing forces between the T-section wire and the U-section wire run the risk of remaining excessively great.
A first prototype was made with the ribs of the U-section wire directed inwardly and with modified dimensional characteristics of the T- and U-section wires. Fatigue testing on the prototype under dynamic conditions and under pressure terminated prematurely because the U-section wire broke.
The present invention relies, in particular, on the discovery, made during the investigation subsequent to this breaking of the U-section wire, that the U-section wire is subjected to greater or lesser bending moments caused by the friction forces which are associated with the radial forces between the T-section wire and the U-section wire.